EUV pattern transfer using graded hardmask

ABSTRACT

Techniques for EUV resist pattern transfer using a graded hardmask are provided. In one aspect, a method of patterning is provided. The method includes: forming a graded hardmask on a device stack; depositing a resist onto the graded hardmask; patterning the resist to form a pattern in the resist having at least one feature; modifying at least one surface region to increase an etch rate of the graded hardmask; transferring the pattern from the resist to the graded hardmask; and transferring the pattern from the graded hardmask to at least one underlying layer of the device stack. A device structure formed by the patterning method is also provided.

FIELD OF THE INVENTION

The present invention relates to extreme ultraviolet (EUV) patterntransfer, and more particularly, to EUV resist pattern transfer using agraded hardmask.

BACKGROUND OF THE INVENTION

Pattern transfer using EUV resist is very challenging due to stochasticdefects and aspect ratio control. When patterning in the sub 40nanometer (nm) pitch scale, resist thickness budget that remains forpattern transfer is very low. During pattern transfer, resist residuecan be removed using an oxygen (O₂)-based plasma process (there are alsoother methods such HBr/O₂, CO₂/CH₄, CF₄ based). See, for example, Tiwariet al., “Characterization of the Descum Process for Various SiliconSubstrates,” Abstract #2123, 224^(th) ECS Meeting October/November 2013(1 page) (hereinafter “Tiwari”), the contents of which are incorporatedby reference as if fully set forth herein. However, resist thicknessloss during residue removal and pattern transfer exacerbates theexisting local thinning of resist lines increasing the risk ofnanobridges and line breaks, especially at a reduced pitch scale. Linebridging is caused by resist residue, whereas line breaks occur due tolocal thinning of resist.

Use of a highly selective hardmask etch helps dealing with resist budgetand local resist thinning. However, this only reinforces defects likebridges and line breaks.

Therefore, improved EUV resist pattern transfer techniques would bedesirable.

SUMMARY OF THE INVENTION

The present invention provides techniques for extreme ultraviolet (EUV)resist pattern transfer using a graded hardmask. In one aspect of theinvention, a method of patterning is provided. The method includes:forming a graded hardmask on a device stack; depositing a resist ontothe graded hardmask; patterning the resist to form a pattern in theresist having at least one feature; modifying at least one surfaceregion to increase an etch rate of the graded hardmask; transferring thepattern from the resist to the graded hardmask; and transferring thepattern from the graded hardmask to at least one underlying layer of thedevice stack.

In another aspect of the invention, another method of patterning isprovided. The method includes: forming a graded hardmask on a devicestack, wherein the graded hardmask includes a carbon-containingmaterial, and wherein a carbon content of the graded hardmask graduallydecreases along a gradient from a top to a bottom of the gradedhardmask; depositing a resist onto the graded hardmask; patterning theresist to form a pattern in the resist having at least one feature;removing resist residue from the patterning using an oxygen (O₂)-basedplasma process that modifies at least one surface region of the gradedhardmask exposed within the at least one feature to modify the at leastone surface region to increase an etch rate of the graded hardmask;transferring the pattern from the resist to the graded hardmask; andtransferring the pattern from the graded hardmask to at least oneunderlying layer of the device stack.

A more complete understanding of the present invention, as well asfurther features and advantages of the present invention, will beobtained by reference to the following detailed description anddrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional diagram illustrating a semiconductor devicestructure having a multilayer device stack and extreme ultraviolet (EUV)lithography stack having a carbon-containing graded hardmask with acarbon gradient and EUV resist formed on a wafer according to anembodiment of the present invention;

FIG. 2 is a cross-sectional diagram illustrating the EUV resist havingbeen patterned to form a pattern in the EUV resist containing at leastone feature, and how randomly formed residue can remain in the featuresafter patterning according to an embodiment of the present invention;

FIG. 3 is a cross-sectional diagram illustrating an oxygen (O₂)-basedcleaning process having been performed to remove the residue whichremoves carbon from the surface of the hardmask and increases porosityaccording to an embodiment of the present invention;

FIG. 4 is a cross-sectional diagram illustrating a hardmask open etchhaving been performed to transfer the pattern from the EUV resist to thehardmask using, e.g., a fluorine-based etch according to an embodimentof the present invention;

FIG. 5 is a cross-sectional diagram illustrating the pattern having beentransferred from the hardmask to one or more of the underlying layers ofdevice stack according to an embodiment of the present invention;

FIG. 6 is a cross-sectional diagram illustrating, according to oneexemplary implementation, the pattern having been filled with a contactmetal according to an embodiment of the present invention;

FIG. 7 is a diagram illustrating an exemplary methodology for patterningusing the present graded hardmask according to an embodiment of thepresent invention; and

FIG. 8 is a diagram illustrating how etch selectivity for the gradedhardmask can increase as the carbon content in hardmask tapers along thegradient according to an embodiment of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Provided herein are improved techniques for extreme ultraviolet (EUV)resist pattern transfer that employ a graded hardmask. For instance, aswill be described in detail below, the hardmask is formed from acarbon-containing material such as silicon oxycarbide (SiOC), siliconcarbide (SiC) and/or silicon carbonitride (SiCN) having a carbon (C)composition gradient with more carbon at the top of the hardmask film.The carbon content of the hardmask film gradually decreases towards thebottom of the film.

As will be described in detail below, use of the present graded hardmaskallows for an oxygen (O₂)-based process for resist residue removal whichalso partially modifies and/or removes the hardmask such that asubsequent etch step can etch the hardmask without consuming too much ofthe resist. Advantageously, this approach allows for more overetchduring the resist residue removal process since it also modifies thehardmask in a way that the subsequent hardmask etch will be moreselective to the resist.

Referring to FIG. 1 , for example, a semiconductor device structure 100is shown having a multilayer device stack 102 and EUV lithography stack104 formed on a wafer 101. It is notable that the various structuresdepicted in the figures are not necessarily drawn to scale.

According to an exemplary embodiment, wafer 101 is a bulk semiconductorwafer, such as a bulk silicon (Si), bulk germanium (Ge), bulk silicongermanium (SiGe) and/or bulk III-V semiconductor wafer. Alternatively,wafer 101 can be a semiconductor-on-insulator (SOI) wafer. A SOI waferincludes a SOI layer separated from an underlying substrate by a buriedinsulator. When the buried insulator is an oxide it is referred toherein as a buried oxide or BOX. The SOI layer can include any suitablesemiconductor, such as Si, Ge, SiGe, and/or a III-V semiconductor.

Device stack 102 disposed on wafer 101 can be composed of a variety ofdifferent layers, the contents of which can depend on the particularapplication. Ultimately, however, the pattern from the EUV lithographystack 104 will be transferred to one or more of the underlying layers indevice stack 102. According to one non-limiting example, device stack102 includes a nitride layer 106, an oxide layer 108 disposed on thenitride layer 106, and an organic planarizing layer (OPL) 110 disposedon the oxide layer 108.

Suitable materials for the nitride layer 106 include, but are notlimited to, silicon nitride (SiN), silicon oxycarbonitride (SiOCN)and/or metal nitrides such as titanium nitride (TiN). Nitride layer 106can be deposited onto the wafer 101 using a process such as chemicalvapor deposition (CVD), physical vapor deposition (PVD) or atomic layerdeposition (ALD). According to an exemplary embodiment, the nitridelayer 106 is deposited to a thickness of from about 20 nanometers (nm)to about 26 nm and ranges therebetween, e.g., about 23 nm.

Suitable materials for the oxide layer 108 include, but are not limitedto, silicon dioxide (SiO₂) and/or a SiO₂ precursor such astetraorthosilicate (TEOS) (Si(OC₂H₅)₄). Oxide layer 108 can be depositedonto nitride layer 106 using a process such as CVD, PVD or ALD.According to an exemplary embodiment, the oxide layer 108 include isdeposited to a thickness of from about 22 nm to about 28 nm and rangestherebetween, e.g., about 25 nm.

According to an exemplary embodiment, OPL 110 contains an aromaticcross-linkable polymer (e.g., naphthalene-based) in a solvent. Othersuitable materials for use in OPL 110 include but are not limited tothose materials described in U.S. Pat. No. 7,037,994 issued to Sugita etal. entitled “Acenaphthylene Derivative, Polymer, and AntireflectionFilm-Forming Composition,” U.S. Pat. No. 7,244,549 issued to Iwasawa etal. entitled “Pattern Forming Method and Bilayer Film,” U.S. Pat. No.7,303,855 issued to Hatakeyama et al. entitled “PhotoresistUndercoat-Forming Material and Patterning Process” and U.S. Pat. No.7,358,025 issued to Hatakeyama entitled “Photoresist Undercoat-FormingMaterial and Patterning Process.” The contents of each of the foregoingpatents are incorporated by reference herein. OPL 110 can be depositedonto the oxide layer 108 using a casting process such as spin-coating orspray coating. Following deposition, a post-apply bake is performed tocross-link the OPL 110 and bake off the solvent. By way of example only,the post-apply bake can be conducted at a temperature of up to about 250degrees Celsius (° C.), e.g., from about 200° C. to about 250° C. andranges therebetween. According to an exemplary embodiment, OPL 110 isformed having a thickness of from about 40 nm to about 80 nm and rangestherebetween, e.g., about 60 nm.

EUV lithography stack 104 includes a graded hardmask 112 disposed on thedevice stack 102 (i.e., on OPL 110), and an EUV resist 114 disposed onthe hardmask 112. As provided above, hardmask 112 is formed from acarbon-containing material such as SiOC, SiC and/or SiCN. Referring tomagnified view 116 of hardmask 112, the carbon (C) content in hardmask112 gradually decreases along a gradient moving from the top to thebottom of hardmask 112. Namely, as shown in magnified view 116, hardmask112 is carbon rich at the top and carbon depleted at the bottom.

By way of example only, the top of hardmask 112 contains from about 4percent (%) to about 25% carbon and ranges therebetween, whereas thebottom of hardmask 112 contains from about 0% to about 1% carbon andranges therebetween. Regions of the hardmask 112 in between the top andthe bottom have a carbon content in between these top and bottom ranges.Further, the carbon content gradually decreases as a function of depthin the hardmask 112 film. For instance, according to an exemplaryembodiment, the carbon content decreases by from about 1% to about 2%and ranges therebetween every from about 1 nm to about 2 nm and rangestherebetween from the top to the bottom of the hardmask 112. Thus, usinga simple, non-limiting example to illustrate this concept, the carboncontent at the top of the film might be 20%. At a depth of 2 nm into thehardmask 112 film the carbon content might drop to 18%, at a depth of 4nm into the hardmask 112 film the carbon content might drop to 16%, andso on.

Notably, the gradient is gradual and hardmask 112 is made up of a singlelayer containing a gradient of the carbon as opposed, for example, tomultiple layers of different materials. Further, as will be described indetail below, surface regions of the hardmask 112 will be modified toincrease porosity and thus an etch rate through the hardmask, whichincreases the (hardmask-to-resist) etch selectivity. Thus, during thehardmask open, the amount of resist that is consumed is minimized.Accordingly, unwanted artifacts such as bridging and line breaks can belargely avoided to maintain the integrity of the pattern in the EUVresist 114. According to an exemplary embodiment, hardmask 112 is formedhaving a thickness of from about 5 nm to about 15 nm and rangestherebetween.

By way of example only, hardmask 112 having a carbon gradient can bedeposited onto the device stack 102 using a controlled CVD, ALD orPVD-based process. See, for example, U.S. Pat. No. 6,429,538 issued toLin, entitled “Method for Making a Novel Graded Silicon Nitride/SiliconOxide (SNO) Hard Mask for Improved Deep Sub-Micrometer SemiconductorProcessing” (a graded composite created using LPCVD and controlling thereactant gases) and Babich et al., “A Novel Graded AntireflectiveCoating with Built-in Hardmask Properties Enabling 65 nm and Below CMOSDevice Patterning,” IEEE International Electron Devices Meeting December2003 (4 pages) (Si:C:O:H materials prepared by PECVD), the contents ofboth of which are incorporated by reference as if fully set forthherein. By way of example only, precursors for the presentcarbon-containing hardmask materials such as SiOC, SiC and/or SiCNinclude, but are not limited to Bistrimethylsilylmethane (BTMSM) and/ortrimethylcyclote-trasiloxane (TMCTS).

Any commercially available EUV resist materials may be employed as EUVresist 114. See, for example, metal oxide EUV resists available fromInpria™, Corvallis, Oreg. Other suitable EUV resist materials aredescribed, for example, in U.S. Patent Application Publication Number2012/0208124 A1 by Iwashita et al., entitled “Resist Composition forEUV, Method for Producing Resist Composition for EUV, and Method ofForming Resist Pattern,” the contents of which are incorporated byreference as if fully set forth herein. A casting process such as spincoating and/or spray coating can be employed to deposit EUV resist 114onto the hardmask 112. According to an exemplary embodiment, EUV resist114 is deposited to a thickness of from about 5 nm to about 10 nm andranges therebetween.

As will be described in detail below, a graded hardmask 112 (e.g., SiOC,SiC and/or SiCN) with more carbon at the top of the hardmask film allowsan O₂-based residue removal process to modify the top layer of hardmask112. The modified hardmask 112 can then be etched relatively easilyusing etch chemistry that is selective to EUV resist 114, therebypreserving more of the EUV resist 114 and thus preventing unwantedoccurrences such as line breaks.

The advantages of the present graded hardmask design are made furtherevident by reference to the exemplary methodology for EUV patterningshown illustrated in FIGS. 2-6 . In FIGS. 2-6 , the semiconductor devicestructure 100 of FIG. 1 is used as a non-limiting example, and likestructures are numbered alike.

Namely, as provided above, semiconductor device structure 100 includes amultilayer device stack 102 and EUV lithography stack 104 formed on a(e.g., bulk semiconductor or SOI) wafer 101. In this example, the devicestack 102 includes a nitride layer 106 (e.g., SiN, SiOCN and/or TiN), anoxide layer 108 (e.g., SiO₂ and/or a SiO₂ precursor such as TEOS)disposed on the nitride layer 106, and an OPL 110 disposed on the oxidelayer 108. The EUV lithography stack 104 includes a graded hardmask 112(e.g., SiOC, SiC and/or SiCN) disposed on the device stack 102, and anEUV resist 114 disposed on the hardmask 112. As provided above, thecarbon (C) composition in hardmask 112 gradually decreases along agradient moving from the top to the bottom of hardmask 112 such that thehardmask 112 is carbon rich at the top and carbon depleted at thebottom. For instance, according to an exemplary embodiment, the carboncontent in hardmask 112 gradually decreases by from about 1% to about 2%and ranges therebetween every from about 1 nm to about 2 nm and rangestherebetween from the top of the hardmask 112.

As shown in FIG. 2 , the process begins with the patterning of EUVresist 114 which involves forming a pattern containing at least onefeature 202 (e.g., trenches) in EUV resist 114. Standardphotolithography techniques can be employed to pattern the EUV resist114.

Following patterning of the EUV resist 114, residue can remain in thefeatures 202 and needs to be removed prior to pattern transfer to thehardmask 112. See FIG. 2 . Namely, as shown in FIG. 3 , a cleaningprocess is next performed to remove the residue. According to anexemplary embodiment, the cleaning process employed is an O₂-basedplasma process (wherein O₂ is the reactive species). During an O₂-basedplasma process, a few hundred angstroms of the EUV resist 114 is removed(see, e.g., Tiwari), including the residue.

Notably, exposure of (graded SiOC, SiC and/or SiCN) hardmask 112 to anO₂ plasma will remove carbon from the surface of hardmask 112, forming aporous SiO₂-like material. See, for example, Shamiryan et al.,“Comparative study of SiOCH low-k films with varied porosity interactingwith etching and cleaning plasma,” J. Vac. Sci. Technol. B 20(5), pp.1923-1928 (September/October 2002) (hereinafter “Shamiryan”), thecontents of which are incorporated by reference as if fully set forthherein. As a result, the surface regions 302 of hardmask 112 exposed tothe O₂ plasma within the features 202 are modified by the O₂ plasma. SeeFIG. 3 . Namely, the surface regions 302 will have increased porosity.For instance, by way of example only, pre-exposure, the hardmask 112 hasa porosity of from about 10% to about 30% and ranges therebetween.Following exposure to the O₂ plasma, a porosity of the surface regions302 increases to from about 20% to about 60% and ranges therebetween.Increasing the porosity increases the etch rate. See Shamiryan. Thus,increasing the porosity of surface regions 302 will increase the etchrate through the surface regions 302 thus increasing the overall etchrate through hardmask 112 during the hardmask open (see below).Advantageously, as highlighted above, increasing the etch rate throughthe hardmask 112 increases the hardmask-to-resist selectivity bydecreasing the amount of EUV resist 114 that is consumed during thehardmask open.

Namely, as shown in FIG. 4 , a hardmask open etch is performed totransfer the pattern (including features 202) from the EUV resist 114 to(graded) hardmask 112. According to an exemplary embodiment, EUV with afluorine-based chemistry is used for the hardmask open etch. By way ofexample only, suitable fluorine-based etches include, but are notlimited to, octafluorocyclobutane (C₄F₈)/O₂ and/or tetrafluoromethane(CF₄)/difluoromethane (CH₂F₂). As provided above, the porosity ofsurface regions 302 of hardmask 112 is increased. An increased hardmaskporosity provides more accessibility to the etch, thereby increasing theoverall etch rate through the hardmask 112.

An increased etch rate through the hardmask 112 results in less of theEUV 114 being consumed during the hardmask open. This is advantageousbecause resist loss results in alterations to the pattern transferred tothe hardmask 112. In extreme cases, bridges and/or line breaks canundesirably occur. For instance, when the resist is overly thinned,barriers between adjacent lines/features can disappear during patterntransfer.

Further, with fluorine-based etches the selectivity for hardmask 112(over EUV resist 114) can actually increase as the carbon content inhardmask 112 tapers along the gradient. See, for example, FIG. 8—described below. Thus, during the hardmask open, the hardmask-to-resistselectivity can increase as the etch progresses through the hardmask 112based on the gradual decrease in carbon content (along the gradient).

Following the hardmask 112 open, the remaining EUV resist 114 is removedand, as shown in FIG. 5 , the pattern is transferred from the(now-patterned) hardmask 112 to one or more of the underlying layers ofdevice stack 102. For instance, by way of example only, the pattern formhardmask 112 is transferred to OPL 110 and oxide layer 108.

The present techniques can be employed for a variety of differentlithographic processes and applications. For instance, according to anexemplary embodiment, the present techniques are employed to form metallines on the wafer 101. See, for example, FIG. 6 . As shown in FIG. 6 ,following pattern transfer from the hardmask 112, the OPL 110 isremoved. The pattern in oxide layer 108 is then filled with a contactmetal 602 such as cobalt (Co), ruthenium (Ru), nickel (Ni) platinum(Pt), palladium (Pd), copper (Cu), etc. Again, what is shown in FIG. 6is merely an example, and the present techniques should not be construedas being limited to any application in particular.

Given the above description, an overview of the present patterningtechniques using EUV lithography with a graded hardmask are summarizedin methodology 700 of FIG. 7 . In step 702, a graded hardmask 112 isformed on a device stack 102 (e.g., nitride layer 106/oxide layer108/OPL 110) disposed on the oxide layer 108. As provided above,according to an exemplary embodiment the graded hardmask 112 is formedfrom a carbon-containing material such as SiOC, SiC and/or SiCN, and thecarbon content of the graded hardmask 112 gradually decreases along agradient from the top to the bottom of the graded hardmask 112.

In step 704, an EUV resist 114 is deposited onto the graded hardmask. Instep 706, the EUV resist 114 is patterned to form a pattern in the EUVresist 114 having at least one feature 202 (e.g., trench).

In step 708, EUV resist 114 residue from the patterning is removed. Asprovided above, according to an exemplary embodiment, the resist residueis removed using an O₂-based plasma process, wherein the O₂-based plasmaprocess modifies at least one surface region of the graded hardmask 112exposed within the at least one feature to remove carbon from the atleast one surface region of the graded hardmask 112 and thereby change aporosity of the graded hardmask 112 in the at least one surface region(i.e., to increase an etch rate of the graded hardmask 112).

In step 710, the pattern is transferred from the EUV resist 114 to thegraded hardmask 112 using EUV lithography with, e.g., fluorine-basedetch chemistry such as C₄F₈/O₂ and/or CF₄/CH₂F₂. In step 712, thepattern from the graded hardmask 112 is transferred to at least oneunderlying layer of the device stack 102.

As highlighted above, the etch selectivity for hardmask 112 (over EUVresist 114) can increase as the carbon content in hardmask 112 tapersalong the gradient. See, for example, FIG. 8 . FIG. 8 is a diagramillustrating the etch selectivity of hardmask 112 to an organic film(i.e., a typical EUV resist material) as a function of carbon content.In this example, a fluorine-based etch chemistry C₄F₈/O₂ was evaluated.As described in detail above, the present graded hardmask designincludes a carbon gradient that gradually decreases moving from the topto the bottom of the hardmask film such that the hardmask is carbon richat the top and carbon depleted at the bottom. In this example, SiOChardmask samples with carbon content ranging from 0% to 25% were tested.As shown in FIG. 8 , the selectivity of the C₄F₈/O₂ etch for the SiOChardmask (over EUV resist materials) increases with a decrease in thecarbon content. Thus, during the hardmask open, the hardmask-to-resistselectivity is enhanced the farther the etch progresses through thehardmask 112 based on the gradual decrease in carbon content along thegradient.

Although illustrative embodiments of the present invention have beendescribed herein, it is to be understood that the invention is notlimited to those precise embodiments, and that various other changes andmodifications may be made by one skilled in the art without departingfrom the scope of the invention.

What is claimed is:
 1. A method of patterning, comprising the steps of:forming a graded hardmask directly on a device stack, wherein the gradedhardmask comprises a carbon-containing material, and wherein the gradedhardmask is made up of a single layer containing a gradient of carbon;depositing a resist directly onto the graded hardmask; patterning theresist to form a pattern in the resist comprising at least one feature;modifying at least one surface region of the graded hardmask to increasean etch rate of the graded hardmask; transferring the pattern from theresist to the graded hardmask; and transferring the pattern from thegraded hardmask to at least one underlying layer of the device stack. 2.The method of claim 1, wherein the carbon-containing material isselected from the group consisting of: silicon carbide (SiC), siliconoxycarbide (SiOC), silicon carbonitride (SiCN), and combinationsthereof.
 3. The method of claim 1, wherein a carbon content of thegraded hardmask gradually decreases along the gradient from a top to abottom of the graded hardmask.
 4. The method of claim 3, wherein the topof the graded hardmask comprises from about 4% to about 25% carbon andranges therebetween, and wherein the bottom of the graded hardmaskcomprises from about 0% to about 1% carbon and ranges therebetween. 5.The method of claim 3, wherein the carbon content decreases by fromabout 1% to about 2% and ranges therebetween every from about 1 nm toabout 2 nm and ranges therebetween from the top to the bottom of thegraded hardmask.
 6. The method of claim 1, wherein the graded hardmaskhas a thickness of from about 5 nm to about 15 nm and rangestherebetween.
 7. The method of claim 1, further comprising the step of:removing resist residue from the patterning.
 8. The method of claim 7,wherein the resist residue is removed using an oxygen (O₂)-based plasmaprocess, wherein the O₂-based plasma process modifies the at least onesurface region of the graded hardmask exposed within the at least onefeature to remove carbon and change a porosity of the graded hardmask inthe at least one surface region.
 9. The method of claim 1, wherein thepattern is transferred from the resist to the graded hardmask usingextreme ultraviolet (EUV) lithography with a fluorine-based etchchemistry.
 10. The method of claim 9, wherein the fluorine-based etchchemistry is selected from the group consisting of:octafluorocyclobutane (C₄F₈)/O₂, tetrafluoromethane(CF₄)/difluoromethane (CH₂F₂) and combinations thereof.
 11. The methodof claim 1, wherein the device stack comprises: a nitride layer; anoxide layer disposed on the nitride layer; and an organic planarizinglayer (OPL) disposed on the oxide layer, wherein the graded hardmask isformed on the OPL.
 12. The method of claim 11, wherein the nitride layercomprises a material selected from the group consisting of: siliconnitride (SiN), silicon oxycarbonitride (SiOCN), a metal nitride,titanium nitride (TiN) and combinations thereof.
 13. The method of claim11, wherein the oxide layer comprises a material selected from the groupconsisting of: silicon dioxide (SiO₂), tetraorthosilicate (TEOS), andcombinations thereof.
 14. A method of patterning, comprising the stepsof: forming a graded hardmask directly on a device stack, wherein thegraded hardmask comprises a carbon-containing material, wherein thegraded hardmask is made up of a single layer containing a gradient ofcarbon, and wherein a carbon content of the graded hardmask graduallydecreases along the gradient from a top to a bottom of the gradedhardmask; depositing a resist directly onto the graded hardmask;patterning the resist to form a pattern in the resist comprising atleast one feature; removing resist residue from the patterning using anO₂-based plasma process that modifies at least one surface region of thegraded hardmask exposed within the at least one feature to modify the atleast one surface region to increase an etch rate of the gradedhardmask; transferring the pattern from the resist to the gradedhardmask; and transferring the pattern from the graded hardmask to atleast one underlying layer of the device stack.
 15. The method of claim14, wherein the carbon-containing material is selected from the groupconsisting of: SiC, SiOC, SiCN, and combinations thereof.
 16. The methodof claim 14, wherein the top of the graded hardmask comprises from about4% to about 25% carbon and ranges therebetween, wherein the bottom ofthe graded hardmask comprises from about 0% to about 1% carbon andranges therebetween, and wherein the carbon content decreases by fromabout 1% to about 2% and ranges therebetween every from about 1 nm toabout 2 nm and ranges therebetween from the top to the bottom of thegraded hardmask.
 17. The method of claim 14, wherein the pattern istransferred from the resist to the graded hardmask using EUV lithographywith a fluorine-based etch chemistry selected from the group consistingof: C₄F₈/O₂, CF₄/CH₂F₂, and combinations thereof.
 18. A devicestructure, formed by a patterning method comprising the steps of:forming a graded hardmask directly on a device stack, wherein the gradedhardmask comprises a carbon-containing material, wherein the gradedhardmask is made up of a single layer containing a gradient of carbon,and wherein a carbon content of the graded hardmask gradually decreasesalong the gradient from a top to a bottom of the graded hardmask;depositing a resist directly onto the graded hardmask; patterning theresist to form a pattern in the resist comprising at least one feature;modifying at least one surface region of the graded hardmask to increasean etch rate of the graded hardmask; transferring the pattern from theresist to the graded hardmask; and transferring the pattern from thegraded hardmask to at least one underlying layer of the device stack.19. The device structure of claim 18, wherein the carbon-containingmaterial is selected from the group consisting of: SiC, SiOC, SiCN, andcombinations thereof.